Fingerprints make it possible to reliably establish the identity of persons. Fingerprint sensors are therefore used to identify people authorized to use credit cards, other types of smart cards, passes and the like. European Patent EP 0 459 808 B1 discloses an example of such a fingerprint sensor which has a matrix of pressure-sensitive cells and triggers switching operations that are based on the lines in the skin of the finger tips or pads. These switching operations are then used for identification by evaluation electronics. In addition, capacitance measuring fingerprint sensors already exist, in which the sensor array records differences in capacitance between finger pad regions which are in contact and those which are not in contact with the sensor. These differences in capacitance then allow reliable identification by evaluation electronics.
For reliably recording fingerprints, the sensor array in a capacitance measuring fingerprint sensor should have a minimum size of approximately 13xc3x9713 mm, and when used in smart cards, the array should also have a certain degree of flexibility. On the other hand, since structure fineness of less than 100 xcexcm is required for such sensor arrays, the corresponding structures are produced on silicon using thin film technology. In this case, such sensor arrays can be made flexible, within certain limits, by grinding down the silicon support, which is cost intensive.
It is accordingly an object of the invention to provide a sensor array for a capacitance measuring fingerprint sensor and a method of producing the sensor array which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type. In particular, it is an object of the invention to provide a sensor array that has a simple design and that can be produced using economical thin film technology.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sensor array for a capacitance measuring fingerprint sensor. The sensor array includes: a base layer made of an electrically insulating material; a plurality of first conductor tracks configured on the base layer; and a first insulation layer made of an electrically insulating material disposed on top of the base layer and the plurality of the first conductor tracks. The first insulation layer has a plurality of plated-through holes formed therein and on top of associated ones of the plurality of the first conductor tracks that are situated underneath. The sensor array also includes: a plurality of first electrodes configured on the first insulation layer and electrically conductively connected to the associated ones of the plurality of the first conductor tracks by the plurality of the plated-through holes; a plurality of second conductor tracks configured on the first insulation layer and crossing the plurality of the first conductor tracks; a plurality of second electrodes configured on the first insulation layer and electrically conductively connected to associated ones of the plurality of the second conductor tracks; and a second insulation layer disposed on the first insulation layer, the plurality of the first electrodes, the plurality of the second conductor tracks, and on the plurality of the second electrodes.
The inventive sensor array permits a simple and reliable measurement principle in which the change in the stray capacitance between two adjacent electrodes as a result of the grooves in the finger skin which are in contact is measured. In this context, the sensor array can be produced using thin film technology on a rigid substrate, or in the form of a flexible fine structure suitable for use in smart cards. The latter option is made possible by the invention""s method for producing a sensor array. This method is based on recognition of the fact that the disadvantages associated with processing flexible support materials can be avoided if a thin base layer made of a flexible organic material can first be applied to a rigid auxiliary support and can be removed from the auxiliary support again once the metal fine structures of the sensor array have been produced, without the risk of damage. Such careful removal of the base layer can be carried out by laser ablation from the back of the auxiliary support, provided that the auxiliary support is made of a material which is at least largely permeable to the laser radiation used. Whereas the conventional production of flexible wiring systems allows only structure dimensions of above 100 xcexcm to be provided, the temporary use of the rigid auxiliary support in this case makes structure finenesses of much less than 100 xcexcm possible.
In accordance with an added feature of the invention, the sensor array is produced on a base layer made of economical organic material.
In accordance with an additional feature of the invention, the sensor array is produced on a base layer made of flexible organic material. Then the sensor array can be used in smart cards or the like without the risk of breakage.
In accordance with another feature of the invention, the base layer is made of a film of a thermostable polyimide. This permits the finished sensor array to be used without any difficulty even at increased ambient temperature.
In accordance with a further feature of the invention, a planar layer made of an electrically insulating material is applied to the base layer. The film can then be provided with a very high surface quality which permits the finest metal structures to be formed.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing a sensor array for a capacitance measuring fingerprint sensor, that includes steps of: providing a rigid auxiliary support made from a material that is at least largely permeable to a given type of laser radiation; applying a thin base layer made of a flexible organic material on the auxiliary support; configuring a plurality of first conductor tracks on the base layer; applying a first insulation layer to the base layer and the plurality of the first conductor tracks; producing holes in the insulation layer; forming, on the first insulation layer, a plurality of first electrodes, a plurality of second conductor tracks crossing the plurality of the first conductor tracks, and a plurality of second electrodes that are electrically conductively connected to associated ones of the plurality of the second conductor tracks; electrically conductively connecting the plurality of the first electrodes to associated ones of the plurality of the first conductor tracks that are situated underneath by providing a metal coating in the holes in the insulation layer; applying a second insulation layer to the first insulation layer, the plurality of the first electrodes, the plurality of the second conductor tracks and the plurality of the second electrodes; and directing the laser radiation of the given type through the auxiliary support onto the base layer to remove the base layer.
In accordance with an added mode of the invention, a quartz glass is used as the material from which the auxiliary support is made; and the laser radiation of the given type is provided from an excimer laser having a laser radiation wavelength of 248 nm. This permits the auxiliary support to have a permeability to laser radiation of approximately 90%.
In accordance with an additional mode of the invention, borosilicate glass is used as the quartz glass for the auxiliary support; and the laser radiation of the given type is provided from an excimer laser having a laser radiation wavelength of 350 nm. This likewise permits the auxiliary support to have a permeability to laser radiation of approximately 90%, but in this case enables the auxiliary support made of borosilicate glass, to be provided at a relatively low cost.
In accordance with another mode of the invention, an adhesive layer is applied to the auxiliary support before the base layer is applied to the auxiliary support. This provides improved adhesion of the base layer to the auxiliary support during processing of the structure.
In accordance with a further mode of the invention, the adhesive layer is made of titanium, which is permeable to the laser radiation when the base layer is removed.
In accordance with a further added mode of the invention, the adhesive layer is applied to the auxiliary support with an extremely low layer thickness by sputtering.
In accordance with a concomitant mode of the invention, the base layer is applied as a film. Thus, the base layer can be applied to the auxiliary support in an extremely simple and economical manner.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a sensor array for a capacitance measuring fingerprint sensor, and method for producing such a sensor array, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.